1. Field of the Invention
The present invention relates in general to Global Positioning System (GPS) receivers, and in particular to resolving time ambiguity in GPS using an over-determined navigation solution.
2. Description of the Related Art
In GPS, there are four variables, namely, position determined by x, y, and z coordinates, and time (x, y, z, and t). These variables are determined by using triangulation techniques and accurate system clocks to determine the location of a GPS receiver through range, range-rate, and pseudo-range measurements made by or at the GPS receiver.
To accurately determine the x, y, z, and t variables, four GPS satellite signals are typically required to provide four simultaneous equations that are solved for the four variables. However, in many environments, such as urban canyons, in buildings, or other weak-signal environments, only three satellites are visible from the GPS receiver. As such, one of the variables must be empirically determined.
It can be seen, then, that there is a need in the art for a method to empirically determine at least one of the GPS variables. It can also be seen, then that there is a need in the art for an apparatus for empirically determining at least one of the GPS variables.
To minimize the limitations in the prior art, and to minimize other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses methods and apparatuses for determining time in a GPS system by overdetermining the position of a mobile GPS receiver. By overdetermining the position to very accurate measurements, the time ambiguity can be resolved to lower error limits than possible without the overdetermination algorithm.
A method for resolving a variable ambiguity in a global positioning system receiver in accordance with the present invention comprises determining a plurality of pseudoranges to a plurality of Global Positioning System satellites, determining a range for each of the pseudoranges, wherein the range gives a maximum and a minimum for each of the pseudoranges, and minimizing a distance from an estimate of user position to the maximum and minimum for the plurality of pseudoranges.
It is an object of the present invention to provide a method to empirically determine at least one of the GPS variables. It is a further object of the present invention to provide an apparatus for empirically determining at least one of the GPS variables.